1. Field of the Invention
The present invention relates to an apparatus and method for encoding/decoding a code, and more particularly to encoding/decoding a space time block code maximizing a diversity gain and a data transmission rate in a mobile communication system using a multiple input multiple output(MIMO) scheme.
2. Description of the Related Art
The primary concern of communication technology is how efficiently and reliably data can be transmitted through selected channels. In next generation multimedia mobile communication systems currently under development, it is essential to enhance system efficiency by employing suitable channel coding schemes for the systems according to high speed communication system requirements for providing various services based on wireless data beyond the basic voice communication.
However, differently from wired channels, wireless channel conditions in mobile Communication systems generate inevitable errors due to several factors such as multi-path interference, shadowing, propagation attenuation, time-varying noises, interference, and fading, thereby causing information loss.
The information loss causes serious distortion, which may degrade overall performance. Generally, to reduce this information loss, a variety of error-control techniques are employed according to channel characteristics, thereby improving reliability of the systems. The most basic scheme in these error-control techniques is a scheme of employing an error-correcting code.
In addition, a diversity scheme is employed to remove the instability of communication due to fading. The diversity scheme is classified into a time diversity scheme and a space diversity scheme such as a frequency diversity scheme or an antenna diversity scheme.
Herein, the antenna diversity scheme represents a scheme of employing multiple antennas divided into a receive antenna diversity scheme with a plurality of receive antennas, a transmit antenna diversity scheme with a plurality of transmit antennas, and a MIMO scheme with a plurality of receive antennas and a plurality of transmit antennas.
The MIMO scheme denotes a kind of a space time coding (STC) scheme which is a preset coding scheme through which coded signals are transmitted via plural transmit antennas, so that the coding in the time domain is expanded to the space domain, thereby achieving a lower error rate. A space time block coding (STBC) scheme (one of schemes suggested in order to efficiently employ the antenna diversity scheme) has been suggested by Vahid Tarokh (reference; Vahid Tarokh, “Space Time Block Coding From Orthogonal Design,” Institute of Electrical and Electronics Engineers(IEEE) Trans. on Info., Theory, Vol. 45, pp. 1456-1467, July 1999). The space time block coding scheme is a scheme expanded such that a transmit antenna diversity suggested by S. M. Alamouti (reference; S. M. Alamouti, “A Simple Transmitter Diversity Scheme For Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1998 ) may be applied to at least two transmit antennas.
Hereinafter, a structure of a transmitter in a MIMO mobile communication system employing four transmit antennas (Tx.ANTs) and the STBC scheme suggested by Vahid Tarokh will be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating the structure of the transmitter in the MIMO mobile communication system using the four transmit antennas (Tx.ANTs) and the STBC scheme suggested by Vahid Tarokh.
The transmitter includes a modulator 100, a serial to parallel (S/P) converter 102, a space time block encoder 104, and four transmit antennas which are a first transmit antenna (Tx.ANT 1) 106 to a fourth transmit antenna (Tx. ANT 4) 112.
When information data bits are input to the modulator 100, the modulator 100 creates modulated symbols by modulating the input information data bits through a preset modulation scheme and then outputs the modulated symbols to the S/P converter 102. Herein, the modulation scheme may be one of a binary phase shift keying (BPSK) scheme, a quadrature phase shift keying (QPSK) scheme, a quadrature amplitude modulation (QAM) scheme, a pulse amplitude modulation (PAM) scheme, a phase shift keying (PSK) scheme, etc.
The S/P converter 102 receives the serial modulated symbols output from the modulator 100, converts the serial modulated symbols into parallel modulated symbols and then outputs the converted symbols to the space time block encoder 104. Herein, it is assumed that the serial modulated symbols output from the modulator 100 are ‘s1s2s3s4’. The space time block encoder 104 outputs modulated symbols in Equation (1) by space time encoding four modulated symbols (s1, s2, s3, S4) received from the S/P converter 102:
                              G          4                =                  [                                                                      s                  1                                                                              s                  2                                                                              s                  3                                                                              s                  4                                                                                                      -                                      s                    2                                                                                                s                  1                                                                              -                                      s                    4                                                                                                s                  3                                                                                                      -                                      s                    3                                                                                                s                  4                                                                              s                  1                                                                              -                                      s                    2                                                                                                                        -                                      s                    4                                                                                                -                                      s                    3                                                                                                s                  2                                                                              s                  1                                                                                                      s                  1                  *                                                                              s                  2                  *                                                                              s                  3                  *                                                                              s                  4                  *                                                                                                      -                                      s                    2                    *                                                                                                s                  1                  *                                                                              -                                      s                    4                    *                                                                                                s                  3                  *                                                                                                      -                                      s                    3                    *                                                                                                s                  4                  *                                                                              s                  1                  *                                                                              -                                      s                    2                    *                                                                                                                        -                                      s                    4                    *                                                                                                -                                      s                    3                    *                                                                                                s                  2                  *                                                                              s                  1                  *                                                              ]                                    (        1        )            
In Equation (1), the G4 denotes a coding matrix for symbols transmitted through four transmit antennas. In the matrix of Equation (1), each element of each row corresponds to time slot, and each element of each column corresponds to each transmit antenna at a corresponding time slot.
That is, at a first time slot, the symbols s1, s2, s3, and s4 are transmitted through the first transmit antenna 106 to the fourth transmit antenna 112, respectively. Similarly, at an eighth time slot, the symbols −s4*, −s3*, s2*, and s1*, are transmitted through the first transmit antenna 106 to the fourth transmit antenna 112, respectively.
As described with reference to Equation (1), the space time block encoder 104 controls input modulated symbols to be transmitted through the four transmit antennas at eight time slots by performing a negative operation and a conjugate operation with respect to the input modulated symbols. Herein, diversity gains corresponding to diversity orders can be acheived because the symbols transmitted through the four transmit antennas are orthogonal to each other.
The structure of the transmitter in the MIMO mobile communication system employing the four transmit antennas (Tx.ANTs) and the STBC scheme suggested by Vahid Tarokh is described with reference to FIG. 1. Hereinafter, a structure of a receiver corresponding to the structure of the transmitter shown in FIG. 1 will be described with reference to FIG. 2.
In FIG. 2, the receiver includes plural receive antennas (e.g., P number of receive antennas; a first receive antenna (Rx. ANT 1) 200 to a Pth receive antenna (Rx. ANT P) 202, a channel estimator 204, a signal combiner 206, a detector 208, a parallel to serial (P/S) converter 210, and a de-modulator 212. Although it is assumed that the number of receive antennas in the receiver is different from the number of the transmit antennas in the transmitter corresponding to the receiver in FIG. 2, it is expected that the number of the receive antennas may be identical to the number of the transmit antennas.
As described with reference to FIG. 1, the signals transmitted through the four transmit antenna in the transmitter are received through the first receive antenna 200 to the Pth receive antenna 202, respectively. Each of the first receive antenna 200 to the Pth receive antenna 202 outputs each of the received signals to the channel estimator 204 and the signal combiner 206.
The channel estimator 204 receives the signals input through the first receive antenna 200 to the Pth receive antenna 202, estimates channel coefficients representing channel gains, and outputs signals to the detector 208 and the signal combiner 206. The signal combiner 206 receives signals input through the first receive antenna 200 to the Pth receive antenna 202 and signals output from the channel estimator 204, combines the signals in order to make receive symbols, and then outputs the receive symbols to the detector 208.
The detector 208 generates hypotheses symbols by multiplying the receive symbols output from the signal combiner 206 by the channel coefficients output from the channel estimator 204, calculates decision statistic with respect to all symbols transmittable in the transmitter using the hypotheses symbols, and detects modulated symbols transmitted in the transmitter through threshold detection so as to output the modulated symbols to the parallel/serial converter 210.
The P/S converter 210 receives and converts parallel modulated symbols output from the detector 208 into serial modulated symbols and then outputs the converted symbols to the de-modulator 212. The de-modulator 212 receives the serial modulated symbols output from the P/S converter 210 and recovers original information data bits by de-modulating the serial modulated symbols through a de-modulation scheme corresponding to a modulation scheme employed for the modulator 100 in the transmitter.
As described above, the space time block coding scheme suggested by S. M. Alamouti does not generate data rate loss and can provide the maximum diversity order identical to the number of the transmit antennas even when the transmitter transmits complex symbols through two transmit antennas.
The structures of both the transmitter and the receiver (described with reference to FIGS. 1 and 2) suggested by Vahid Tarokh through expansion of the space time block coding scheme suggested by S. M. Alamouti can provide the maximum diversity order using the space time block code in a type of a matrix having mutually-orthogonal columns. In addition, the structures of both the transmitter and the receiver described with reference to FIGS. 1 and 2 reduce a data rate by half because four complex symbols are transmitted at eight time slots. In addition, the structures degrade receive performance under a fast fading condition because eight time slots are required to transmit one signal block (i.e., four symbols).
When signals are transmitted through at least four transmit antennas based on the space time block coding scheme as described above, transmission of N symbols requires 2× n number of time slots, thereby increasing latency and generating data rate loss.
To design a scheme having a full rate in a multiple antenna communication system transmitting signals through at least three transmit antennas, the Giannakis group suggests a full diversity full rate (FDFR) STBC scheme using four transmit antennas based on constellation rotation in a complex field.
Hereinafter, a structure of a transmitter in a MIMO mobile communication system using four transmit antennas and the STBC scheme suggested by the Giannakis group will be described with reference to FIG. 3.
In FIG. 3, the transmitter includes a modulator 300, a pre-encoder 302, a space time mapper 304, and four transmit antennas (a first transmit antenna (Tx.ANT 1) 306 to a fourth transmit antenna (Tx. ANT 4) 312. If information data bits are input, the modulator 300 generates modulated symbols by modulating the information data bits based on a preset modulation scheme and then outputs the modulated symbols to the pre-encoder 302. Herein, the modulation scheme may be employed as one of a BPSK scheme, a QPSK scheme, a QAM scheme, a PAM scheme, and a PSK scheme.
The pre-encoder 302 receives the four modulated symbols (i.e., d1,d2, d3,d4) output from the modulator 300, encodes the symbols such that signal rotation may occur in a signal space, and outputs the encoded signals to the space time mapper 304. Herein, it is assumed that an input modulated symbol stream including the four modulated symbols output from the modulator 300 is referred to as ‘d’. The pre-encoder 302 generates a complex vector ‘r’ by performing an operation of Equation (2) with respect to the input modulated symbol stream d and outputs the vector r to the space time mapper 304:
                    r        =                              Θ            ⁢                                                  ⁢            d                    =                                                    [                                                                            1                                                                                      α                        0                        1                                                                                                            α                        0                        2                                                                                                            α                        0                        3                                                                                                                        1                                                                                      α                        1                        1                                                                                                            α                        1                        2                                                                                                            α                        1                        3                                                                                                                        1                                                                                      α                        2                        1                                                                                                            α                        2                        2                                                                                                            α                        2                        3                                                                                                                        1                                                                                      α                        3                        1                                                                                                            α                        3                        2                                                                                                            α                        3                        3                                                                                            ]                            ⁡                              [                                                                                                    d                        1                                                                                                                                                d                        2                                                                                                                                                d                        3                                                                                                                                                d                        4                                                                                            ]                                      =                          [                                                                                          r                      1                                                                                                                                  r                      2                                                                                                                                  r                      3                                                                                                                                  r                      4                                                                                  ]                                                          (        2        )            
In Equation 2, the Θ denotes a pre-encoding matrix. The space time block coding scheme suggested by the Giannakis group employs a Vandermonde matrix (unitary matrix) as the pre-encoding matrix. In addition, the αi in Equation (2) is expressed as Equation (3):αi=exp(j2π(i+¼)/4), i=0,1,2,3
. . . (3)The STBC scheme suggested by the Giannakis group is not only proper for the case of using four transmit antennas as described above but also can be easily applied to a case of using more than four transmit antennas. The space time mapper 304 receives and space-time encodes signals output from the pre-encoder 302 and then outputs the signals as modulated symbols in Equation (4):
                    S        =                  [                                                                      r                  1                                                            0                                            0                                            0                                                                    0                                                              r                  2                                                            0                                            0                                                                    0                                            0                                                              r                  3                                                            0                                                                    0                                            0                                            0                                                              r                  4                                                              ]                                    (        4        )            
In Equation (4), the S denotes a coding matrix for symbols transmitted through four transmit antennas. In the matrix of Equation (4), each element of each row corresponds to time slot, and each element of each column corresponds to each transmit antenna at a corresponding time slot.
In other words, at a first time slot, the symbol r1 is transmitted through a first transmit antenna 306, and no signal is transmitted through remaining transmit antennas (i.e., a second transmit antenna 308 to a fourth transmit antenna 312). Similarly, at a fourth time slot, the symbol r4 is transmitted through the fourth transmit antenna 312 and no signal is transmitted through remaining transmit antennas (i.e., the first transmit antenna 308 to the third transmit antenna 310).
The symbols in Equation (4) are received in a receiver (not shown) through a wireless channel. The receiver recovers the modulated symbol stream d through a maximum likelihood (ML) decoding scheme. As a result, the receiver recovers information data bits.
The Tae Jin Jeong and Gyung Hoon Jeon research team suggested a concatenated code and a pre-encoder having a coding gain superior to that of the space time block coding scheme suggested by the Giannakis group in 2003. The Tae Jin Jeong and Gyung Hoon Jeon research team greatly improved by performing concatenation with respect to the space time block code suggested by S. M. Alamouti instead of using a diagonal matrix suggested by the Giannakis group.
Hereinafter, a structure of a transmitter in a MIMO mobile communication system using four transmit antennas and a STBC scheme suggested by the Tae Jin Jeong and Gyung Hoon Jeon research team will be described with reference to FIG. 4.
FIG. 4 is a block diagram illustrating the structure of the transmitter in the MIMO mobile communication system using the four transmit antennas and the STBC scheme suggested by the Tae Jin Jeong and Gyung Hoon Jeon research team.
The transmitter includes a pre-encoder 400, a mapper 402, a delay unit 404, Alamouti encoders 406 and 408, and a first transmit antenna (Tx.ANT 1) 410 to a fourth transmit antenna (Tx.ANT 4) 416. If information data bits are input, the pre-encoder 400 receives four modulated symbols, encodes the modulated symbols such that signal rotation may occur in a signal space, and then outputs the encoded symbols to the mapper 402. Herein, it is assumed that an input modulated symbol stream including the four modulated symbols is referred to as ‘d’. The pre-encoder 400 receives the input modulated symbol stream d and can pre-encode the input modulated symbol stream d as shown in Equation (5):
                    r        =                              Θ            ⁢                                                  ⁢            d                    =                                                    [                                                                            1                                                                                      α                        0                        1                                                                                                            α                        0                        2                                                                                                            α                        0                        3                                                                                                                        1                                                                                      α                        1                        1                                                                                                            α                        1                        2                                                                                                            α                        1                        3                                                                                                                        1                                                                                      α                        2                        1                                                                                                            α                        2                        2                                                                                                            α                        2                        3                                                                                                                        1                                                                                      α                        3                        1                                                                                                            α                        3                        2                                                                                                            α                        3                        3                                                                                            ]                            ⁡                              [                                                                                                    d                        1                                                                                                                                                d                        2                                                                                                                                                d                        3                                                                                                                                                d                        4                                                                                            ]                                      =                          [                                                                                          r                      1                                                                                                                                  r                      2                                                                                                                                  r                      3                                                                                                                                  r                      4                                                                                  ]                                                          (        5        )            
In Equation (5), the αi is equal to ‘αi=exp(j2π(i+¼)/4), i=0, 1, 2, 3’. The mapper 402 receives signals output from the pre-encoder 400 and outputs vectors formed based on two elements ([r1,r2], [r3,r4]). In other words, the mapper 402 outputs ([r1,r2]T) and ([r3,r4]T).
The ([r1,r2]T) is input to the Alamouti encoder 406, and the ([r3,r4]T) is input to the delay unit 404. The delay unit 404 delays the ([r3,r4 ]T) by one time duration and then outputs the delayed ([r3,r4]T) to the Alamouti encoder 408. Herein, the Alamouti encoder represents an encoder employing the space time block coding scheme suggested by S. M. Alamouti.
The Alamouti encoder 406 controls the ([r1,r2]T) output from the mapper 402 to be transmitted through the first transmit antenna 410 and the second transmit antenna 412 at a first time slot. The Alamouti encoder 408 controls the ([r1,r2]T) output from the mapper 402 to be transmitted through the third transmit antenna 414 and the fourth transmit antenna 416 at a second time slot. A coding matrix used for transmitting output signals of the Alamouti encoders 406 and 408 through multiple antennas is expressed as Equation (6):
                    S        =                  [                                                                      r                  1                                                                              r                  2                                                            0                                            0                                                                                      -                                      r                    2                    *                                                                                                r                  1                  *                                                            0                                            0                                                                    0                                            0                                                              r                  3                  *                                                                              r                  4                  *                                                                                    0                                            0                                                              -                                      r                    4                    *                                                                                                r                  3                  *                                                              ]                                    (        6        )            
The coding matrix shown in Equation (6) differs from the coding matrix shown in Equation (4) in that the coding matrix shown in Equation (6) is not a diagonal matrix, but is realized through an Alamouti scheme. The space time block coding scheme suggested by the Tae Jin Jeong and Gyung Hoon Jeon research team increases a coding gain as compared with that of the space time block coding scheme suggested by the Giannakis group by employing a transmission type based on the Alamouti scheme.
However, when the space time block coding scheme suggested by the Tae Jin Jeong and Gyung Hoon Jeon research team is employed, a receiver must perform an operation with respect to all possible elements output from a pre-encoder to recover information data bits transmitted in the transmitter. For example, when the number of transmit antennas is four, the operation must be performed with respect to all 16 elements and there are no elements having the value of zero. That is, in the receiver, loads due to a computation amount increase because the information data bits transmitted in the transmitter are recovered through the ML decoding scheme.
Accordingly, it is desirable to provide an apparatus and method for space time block coding, that minimizes complexity and computation with full diversity full rate.